Secagem convectiva da folha de Aroeira (Schinus terebinthifolius Raddi) em tambor rotativo com parede perfurada: influência das condições operacionais na preservação da cor e compostos bioativos

Abstract

This study investigated drying aroeira (Schinus terebinthifolius Raddi) leaves in a rotating dryer with a perforated wall to evaluate the influence of operating conditions in the preservation of color, and bioactive compounds of leaves. The fresh leaves were characterized through the measurement of dimensions, moisture content, color attributes (L*, a*, and b*), the concentration of total phenolic compounds (CFT), and antioxidant activity (AA). In drying tests performed both in a natural convection oven and in a thin-layer fixed-bed, under temperatures of 50 and 70°C and air velocities of 0.3 and 0.9 m/s, it was verified a significant influence of the air temperature on the drying rate and that internal mass transfer mechanisms control the drying process. The drying conditions in the rotating drum was evaluated through a fractional 2(4-1) experimental design. The air temperature and velocity, the charge (mass) of leaves in the drum, and the drum rotation speed were the investigated factors in this design. The results showed that the drying rate in the rotating drum was strongly dependent on the temperature and, in some conditions, it also depended on the air velocity and the charge of leaves. It was found from the ANOVA that the air temperature was the only statistically significant variable (p-value<0.1) to estimate the drying time and a*/b* ratio (which quantifies the leaf color deterioration). Experimental curves of moisture content versus time were fitted to empirical drying kinetic equations. From the tested equations, the Midilli was the one that best fit the experimental data (R²>0.9924; RMSE<0.0321 and X²<0.0010). Based on the results of the fractional design, a Central Composite Rotational Design (DCCR) was carried out to evaluate the influence of operating variables on the drying time and quality attributes. The assays were conducted under a fixed rotating speed (3 rpm), air temperature varying between 50 and 60°C, air velocity between 0.3 to 0.9m/s, and charge of leaves between 100 and 200g. A decrease in the CFT and AA and an increase in the ratio a*/b* were observed. Regression analysis showed that the temperature, air velocity, and mass of leaves were statistically significant variables for the drying time and a*/b* ratio. The statistical models showed optimal regions laying between Tar > 55°C, var > 0.6 m/s combined with m < 120g, for shorter drying times and, overall, Tar < 53°C for less deterioration. The optimization based on the desirability function indicated that the optimal condition for the shortest drying time (390 min) and the least color deterioration (-0.0782) was achieved under a temperature of 52.3°C, air velocity of 0.86 m/s and charge of leaves of 181.67g.